A retardation film, such as an optical compensation film for optical compensation, is used in a liquid crystal display device in order to make compensation for light that should not be originally transmitted, which is based on optical strain of the liquid crystal display device or on some other factor, and improve optical performances of the liquid crystal display device, such as the viewing angle, the contrast and the color tone thereof. Examples of the retardation film include a film produced by stretching a polymer film monoaxially or biaxially, thereby orienting the main chain of the polymeric compound into a predetermined direction to cause the film to have birefringence; and a film wherein a liquid crystal compound is aligned or oriented on a base film.
In the case of adopting a form in which a liquid crystal compound is aligned on a base film, usually, an alignment film having a function of regulating the alignment direction of the liquid crystal compound is formed on the base film and the liquid crystal compound is aligned on the alignment film. It is known that polyimide, polyvinyl alcohol or some other polymer has alignment property. It is known that a layer of the polymer is formed on a base film and then the resultant is subjected to orienting treatment such as rubbing treatment, thereby forming an alignment film. An example of an aligning method without using rubbing treatment is a method of forming an alignment film by photodimerization reaction, photodecomposition reaction, or photoisomerization reaction. For example, Non-Patent Document 1 discloses a method of controlling the direction along which crosslinked bonds are generated in a polyvinyl cinnamate alignment film by irradiating with polarized ultraviolet rays, and Non-Patent Document 2 discloses a method of causing decomposition reaction of a polyimide alignment film to have anisotropy by irradiating with ultraviolet rays. Besides, it is known that an alignment film is formed by subjecting an inorganic compound to oblique evaporation. However, any alignment film of a polymer layer is more preferred, considering productivity.
However, most of the latest optical devices use a plastic film as a substrate. Accordingly, when an alignment film-forming coating solution is coated on a substrate in order to form an alignment film of a polymer layer on the substrate, the substrate surface may be dissolved in a solvent in the coating solution depending on the kind of the substrate so that additives, such as a plasticizer, in the substrate may elute out into the alignment film. There is a problem that, in such a case, when an optically functional layer is formed on the alignment film, the alignment of a liquid crystal therein may be inhibited. For this reason, it is suggested that a protective layer is formed between an alignment film and a substrate, thereby expressing liquid crystal alignment property stably (for example, Patent Documents 1 to 4).
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-305301
Patent Document 2: Japanese Patent Application Laid-Open No. 2002-6136
Patent Document 3: Japanese Patent Application Laid-Open No. 2002-90532
Patent Document 4: Japanese Patent Application Laid-Open No. 2002-236216
Non-Patent Document 1: M. Schdt et al, JPN. J. Appl. Phys, 31, p 2155-2164 (1992)
Non-Patent Document 2: M. Nishikawa et al, Liquid Crystals, 26, p 575-580 (1990)